I changed from a 11kg cyclocross bike to a 7.5kg carbon bike, tyres on both were 25cc conti gatorskins. On a standard 60-80 mile hilly ride it increased my overall average by about 1.5mph. Big increase in pace uphill, unfortunately a decrease in pace downhill!

Setting: Sheffield and Chesterfield, United Kingdom, between mid-January 2010 and mid-July 2010.

Participants: One consultant in anaesthesia and intensive care.

Main outcome: measure Total time to complete the 27 mile (43.5 kilometre) journey from Sheffield to Chesterfield Royal Hospital and back.

Results: The total distance travelled on the steel frame bicycle during the study period was 809 miles (1302 km) and on the carbon frame bicycle was 711 miles (1144 km). The difference in the mean journey time between the steel and carbon bicycles was 00:00:32 (hr:min:sec; 95% CI –00:03:34 to 00:02:30; P=0.72).

Conclusions: A lighter bicycle did not lead to a detectable difference in commuting time. Cyclists may find it more cost effective to reduce their own weight rather than to purchase a lighter bicycle.

I'm with ahwiles on this. Can't believe there'll be much difference. I found the difference in speed on the road between my road bike (10 kg, 23 mm tyres at 90 psi) and my MTB (11 kg, 2" knobblies - Specialized Fast Trak at 28 psi)is 3 km/h. That's over quite a few long, steady, not-too-hilly rides with the same average heartrate. Probably 15 hrs on each bike. This doesn't account for the automatic 'new bike' speed boost though.

How much does the feel good factor effect how you feel about the ride though? Training more is the best way to go faster, but buying anew bike might well give you much greater enjoyment from your cycling.

it is generally accepted that downhill, a heavier vehicle will be faster than a lightweight vehicle (if cd is equal) so it stands to reason that uphill the opposite will be true, a lighter vehicle will be faster for a given power output.
on the flat, weight is less of a factor, compared to power output vs aero effiency.
so a lighter bike will be quicker uphill, may not be quite as fast downhill, and wont be any faster on the flat if it has the same riding position. (although it will accelerate more quickly)
however, TTIPWP.